Laser cavity configuration yielding dual output beam

ABSTRACT

A laser cavity configuration which enables a single laser medium (gas, liquid or solid) to give an output at least one wavelength from one end of the laser and another output at different wavelengths from the other end of the laser. Disclosed are several well known laser media which are capable of simultaneous laser oscillation at two or more wavelengths. Mirrors which are highly reflective at one of such wavelengths and highly transmissive at another of such wavelengths can be prudently selected to resonate the emission from an active medium. If each of the two mirrors thus required are optimized for reflectivity at one of the wavelengths and transmissivity at the other, the desired effect will result.

United States Patent [191 Willett et al.

[4 1 Nov. 13, 1973 LASER CAVITY CONFIGURATION YIELDING DUAL OUTPUT BEAMInventors: Colin S. Willett, Keedysville; John S.

Kruger, Rockville, both of Md.

The United States of America as represented by the Secretary of theArmy, Washington, DC.

Filed: June 30, 1972 Appl. No.: 267,885

Assignee:

References Cited UNITED STATES PATENTS 1/1970 Zitter 331/945 PrimaryExaminer-William L. Sikes Att0rney-Harry M. Saragovitz et al.

[57] ABSTRACT A laser cavity configuration which enables a single lasermedium (gas, liquid or solid) to give an output at least one wavelengthfrom one end of the laser and another output at different wavelengthsfrom the other end of the laser. Disclosed are several well known lasermedia which are capable of simultaneous laser oscillation at two or morewavelengths. Mirrors which are highly reflective at one of suchwavelengths and highly transmissive at another of such wavelengths canbe prudently selected to resonate the emission from an active medium. Ifeach of the two mirrors thus required are optimized for reflectivity atone of the wavelengths and transmissivity at the other, the desiredeffect will result.

3 Claims, 1 Drawing Figure 1. LASER CAVITY CONFIGURATION YIELDING DUALOUTPUT BEAM RIGHTS OF THE GOVERNMENT The invention described. herein maybe manufactured, used, and licensed. by or for the United StatesGovernment for governmental purposes without the payment to us of anyroyalty thereon.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to lasers, and more particularly to a. laser cavityconfiguration that enables dual simultaneous emission of differentwavelength laser beams from the same active. medium.

2. Description of the Prior Art The availability of a multiplicity ofdifferent wavelength outputs from a single lasing medium has beenheretofore recognized; However, the method of coupling out the two ormore. beams has been less than.

completely satisfactory/The reflectivities of the mirrors utilized toenclose a laser cavity must be carefully selected in order to sustainlaser oscillation. Obviously, when a different wavelength is desired asan output from the same laser medium, the reflectivities of the. mirrorsmustbe. changed in order to sustain oscillation at the new wavelength.When integral mirrors which are permanently affixed to the laserplasmatube are utilized, as is ever increasingly the case, thechangeover to mirrors of different reflective properties becomesextremely difficult and expensive to accomplish. An alternative to therequirement of changing mirrors is illustrated in U.S. Pat. No.3,599,117, wherein a. compound mirror is illustrated having one portionthereof specifically designed to reflect one wavelength and a concentricouter portion specifically designed to reflect the other wavelength.present in the dual beam emission. Unfortunately, the compound mirror sodescribed tends to provide widely divergent output beams which is mostundesirable in certain precision applications.

It is therefore oneobject of the present invention to provide a lasercavity configuration thatallows the. dual simultaneous emission of twolaser beams of differing wavelengths that eliminates the necessity ofchanging the mirrors of the resonant cavity to produce the desiredresult.

Another object of the present invention is to provide a cavityconfiguration that allows dual simultaneous emission of differingwavelength laser beams without the need for compound mirrors whereby thedesired beam width can be more accurately'controlled.

An additional object of the present invention is to provide a lasercavity configuration that permits two laser beams at differingwavelengths to be emitted from the same lasing. medium that isstraightforward, simple to construct and maintain, and producesthedesired result without expensive calibration or alignmentprocedures.

A still further object of the present invention is to provide a duallaser output at two or more different wavelengths that is asymmetricalin which in one embodiment one output is emitted from one end of a lasercavity, whereas the second output is emitted from the opposite end ofthe laser cavity.

SUMMARY OF THE INVENTION ity configuration is provided that allows theemission of two laser beams at differing wavelengths, which comprises anactivelaser medium with appropriate excitation means, said active mediumcapable of laser osciallation at atleasttwo wavelengths, and at least apair of mirrors for reflecting the stimulated radiation back and forththe required number of times to sustain laser oscillation and emission.In a preferred embodiment, two mirrors are selected to have nearlyinverse reflectivity characteristics, depending on the feedbacknecessary to sustain oscillation of the two different wavelengths underconsideration. In other words, one mirror will have a high reflectivityat one wavelength and a low reflectivity at the second wavelength,whereas the mirror at the other end of the plasma tube will have a highreflectivity at the second wavelength and a low reflectivity at thefirst wavelength. By a suitable choice of the reflectivity of thecoatings on both cavity mirrors, it is possible to obtain two wavelengthoutputs with both superradiant laser transitions and low gain lasertransitions, either in pulsed or continuous systems. The requirement isbasically that the reflectivities be high enough to yield simultaneouslaser'oscillation at both desired wavelengths, yet allow sufficientradiation to be coupled out.

BRIEF DESCRIPTION OF THE DRAWING The specific nature of the invention aswell as other objects, aspects, uses, and advantages thereof willclearly appear from the following description and from the accompanyingdrawing, in which:

The FIGURE is a partially schematic side view of a laser cavityconfiguration embodying the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the device of the presentinvention, an asymmetrical laser output is possible in either asuperradiant or low-gain laser system which will yield a laser beam ofat least one wavelength from one end of the laser (under optimum outputcoupling) and another laser beam of differing wavelengths from the otherend. The basic. requirement is that the output coupling of the mirrorsat either end be chosen to be an optimum for the particular wavelengthsemitted therefrom. The basic cavity arrangement is straightforward andis illustrated. in the FIGURE by a preferred embodiment wherein 10represents a laser cavity structure containing an active laser mediuml2.that may be a gas, liquid or solid. If the active medium is gaseous,cavity structure lorepresents a plasma tube, at each end of which aredisposed Brewsters windows 22 and 24. Located outside windows 22 and 24are mirror blanks 1M and 16, respectively, whichhave mirror coatings 20and 18, respectively, located thereon. Laser medium 12 is selected to bea material that is known to lase at at least two differing wavelengths,say A, and A Mirror 16 has coating which has a high reflectivity atwavelength A but lower reflectivity at the other possible laserwavelength A Additionally, mirror 14 has a coating 20 havinga highreflectivity at wavelength A, and a lower reflectivity at wavelength AThe respective beams at wavelengths k, and M will therefore exit throughthe mirror exhibiting the lower reflectivity at their respectivewavelengths. Thus,.the laser beam 28 having wave length A, will exit viamirror 16, whereas the laser beam 26 having wavelength k will exit viaits lower reflectivity mirror 14. Coatings l8 and 20 of laser mirrors 16and 14, respectively, that will satisfy the requirements of highreflectivity at one wavelength and low reflectivity at another can befabricated by an ordinarily skilled technician in the art.

The aforedescribed system finds particular use in high gain,superradiant laser systems. These pulsed gas lasers exhibitsuperradiance as a result of their extremely high gain, which can run ashigh as 1,000,000 per meter of discharge. These lasers are characterizedby a long-lived lower laser level and an upper laser level that ispreferentially excited and is connected to the ground state by a Strongultraviolet or resonance transition. Typical of materials that oscillatesuperradiantly are helium, neon, argon, krypton, xenon, lead, copper,calcium, thallium, strontium, manganese, and nitrogen. Theseself-terminating lasers require short excitation pulses having risetimes on the order of a few to tens of nanoseconds and can be operatedwithout any feedback, but work best with one mirror highly reflectingand another mirror (the output mirror) having a reflectivity of aslittle as 10 percent. Because of the high transmission of the outputmirror and the high reflectivity of the other cavity mirror, if thematerial is chosen to be one under which a dual wavelength oscillationis possible, then simultaneous laser oscillation at the two or morewavelengths will occur in the device of the present invention. Examplesof the foregoing would be found in a neon-nitrogen laser operating inthe visiblespectrum at 5401-A. and 337l-A.; in a neon-hydrogen laser togive an output from one end of the cavity at a wavelength of 486l-A. andat the other end at 4340A.; in a pure neon laser, output at 5401-A.would be achievable at one end and 5944-A. (or 6143-A.) would beavailable from the other end.

The particular laser configuration of the present invention as well asworking with superradiant laser transitions, makes it possible, by asuitable choice of the reflectivity of the coatings on both cavitymirrors, to obtain two wavelength outputs with low gain lasertransitions. The requirement is simply that the reflectivities be highenough to yield simultaneous laser oscillation at both desiredwavelengths. The configuration can be utilized even with lasertransitions which compete for upper laser level populations. An examplecan be found in the common 0.633 ;;.m (red) and the 3.39 pm (infrared)helium-neon laser lines. Orindarily, the efficiency f r illa at 33amistqsl yhsnthq high gain 3.39 pm line is allowed to lase. However,when utilizing the device of the present invention, both outputs can besimultaneously optimized by careful selection of combined mirrorreflectivities. For example, with respect to the FIGURE, mirror 16 andits coating 18 would be optimized for output reflectivity between 95 and99.9 percent at 0.633 urn, whereas its reflectivity at 3.39 p.111 can beas low as to percent. Mirror blank 14 and its coating 20, on the otherhand, could have a 100 percent reflectivity at 0.633 p.m and around 10percent reflectivity at 3.39 pm. Mirror blank 14 can be made of fusedsilica which has special transmissive properties in the infrared regionof the spectrum. Thus, the 3.39 pm beam, represented by 26, will beemitted via mirror 14, and the 0.633 pm beam, represented by 28, willexit via mirror 16. The low reflectivity of both mirrors with respect tothe infrared laser line is the result of the greater feedback necessaryto stimulate the emission of the red line.

Continuous outputs are also realizable from the cavity configuration ofthe present invention. Examples include the 0.633 um line from the 3s 2ptransition and the 1.15 m line from the 2s 2p transition in neon. In theforegoing example, the reflectivities of the mirror coatings can beoptimized to give one of the wavelengths from one end of the lasercavity and the other wavelength from the opposite end, inasmuch as thetwo lines do not compete for upper laser level populations. Having acommon lower laser level does not affect their simultaneous operation.Additionally, since the wavelengths are not in a 2:1 ratio, doubleoperation at the two wavelengths is possible with the mirror coatingsbeing optimized for their respective wavelength reflectivities. Thehelium-neon mixture ratio can be made about 7:1 to make it suitable forproducing population inversion at the two wavelengths. Another possiblecombination of continuous laser outputs would be available from thehelium-xenon laser operating at 2.02 and 3.5 gm simultaneously. The gainin the transitions involved can be high (supperradiant), and it is notedthat these lines do not compete with one another. This combination wouldbe especially apposite in the device of the present invention, inasmuchas the 2.02 pm line usually has lower gain than the 3.5 nm line; thus,the reflectivity at 2.02 gm on the 3.5 pm output mirror must be higherthan the reflectivity at the 2.02 pm output mirror.

The configuration of the present invention can be utilized forcontinuous metal vapor lasers such as the helium-cadmium laser whichwould then yield a blue (0.4416 am) or ultraviolet (0.3250 um) outputfrom one end of the laser cavity, and a green (0.5378 and/or 0.5338 m)or ultraviolet (0.3250 p.m) output from the other end. Another suitablecombination of wavelengths would be a red output (0.6355 m) at one endand the green lines mentioned above at the other end.

One attendant advantage of the configuration as described above is theelimination of the necessity of changing mirrors to get different outputwavelengths from a given plasma. This is particularly significant whenintegral mirrors are permanently affixed to the plasma tube. Ourtechnique allows one output to be taken from the other end of the laserwhenever its wavelength is desired alone. It is quite evident thatunless the gains for the lines on which oscillation is desired are verylow, the technique has a wide application to continuous lasers as wellas to pulsed and/or superradiant lasers. The configuration is no meansrestricted to gas lasers, but can be utilized for all laser media withfeedback mirrors and reflectivities properly chosen. The technique canbe extended to more than two output mirrors with different reflectivitycoatings on each when more than two laser lines can oscillate in thesame medium, provided the separations between lines can fit theseparations between low and high reflectivity bands of mirror coatings.

Additionally, this device will enable the output of one laser beam to bemonitored utilizing the other. This means that one wavelength can bechosen as the monitor wavelength and the other used as the output beamwithout any necessity to sample the output beam directly, provided, ofcourse, that their intensities are related. In this manner, the monitorwavelength can be chosen to match the peak response of the bestavailable detector.

It is to be understood that we have illustrated the principles of thepresent invention primarily by preferred embodiments showing theemission of dual beams at only two differing wavelengths. It is wellunderstood by those versed in the art that more than two wavelengthswill be easily obtainable in output beams of the present invention bycorresponding adjustment in the mirror coating reflectivities.Therefore, .we wish it to be understood that we do not desire to belimited to the exact details of construction shown and described, forobvious modifications will occur to a person skilled in the art.

We claim as our invention:

1. A cavity configuration for stimulating the emission of radiation andfor allowing the simultaneous oscillation and subsequent emission offirst and second laser beams having at least first and secondwavelengths respectively, comprising an active laser medium and firstand second means for reflecting said stimulated radiation back withinsaid medium, said reflecting means located one at each end of saidactive medium which together therewith define a resonant cavity, saidfirst reflecting means having a higher reflectivity at said firstwavelength than said second reflecting means, and said a secondreflecting means having a higher reflectivity at terial.

1. A cavity configuration for stimulating the emission of radiation andfor allowing the simultanEous oscillation and subsequent emission offirst and second laser beams having at least first and secondwavelengths, respectively, comprising an active laser medium and firstand second means for reflecting said stimulated radiation back withinsaid medium, said reflecting means located one at each end of saidactive medium which together therewith define a resonant cavity, saidfirst reflecting means having a higher reflectivity at said firstwavelength than said second reflecting means, and said second reflectingmeans having a higher reflectivity at said second wavelength than saidfirst reflecting means, whereby said first laser beam will be emittedfrom said cavity through said second reflecting means and said secondbeam will be emitted from said cavity through said first reflectingmeans.
 2. The invention according to claim 1 wherein said active lasermedium is selected from the group consisting of neon, nitrogen,hydrogen, argon, helium, lead, copper, mercury, xenon and cadmium. 3.The invention according to claim 1 wherein said active laser mediumcomprises a superradiant laser material.